Automated group gears have been known for a while. They are preferably used in such commercial vehicles where a high number of gears and a large total spread are required. Dependent on the desired number of gear steps, they often comprise a multi-gear main gearbox in lay-shaft design and they also have, in this flow of force, a following two-step range group in a planetary design and/or, upstream in the flow of force, a split group in lay-shaft design. The group configuration, compared to the single gearbox, enables with a comparable number of gears clearly more compact dimensions and a lower weight. Since, depending on the gear change, simple shifting is possible in a gear group and also relatively complicated combinations of gear ratio step changes in several partial gearboxes, shifting takes place preferably fully automated or partially automated. As automated shift transmissions, these group gear boxes offer, compared to manual shift transmissions, increased operating comfort while avoiding operating errors.
In a general shift sequence with gear ratio changes in partial gearboxes, the shift phases in the partial gearboxes take place sequentially, wherein after the load reduction at the drive motor the main gearbox is shifted into neutral, thereafter another partial gearbox will be released and after the other partial gearbox has shifted, the gear in the main transmission is shifted and is synchronized by means of a synchronizer.
However, the inertia of the rotating masses of the gearbox, which need to be synchronized during gear changes, create relatively long shift durations. Long shift durations develop in particular in a range shift where first a range group needs to be shifted between a lower and an upper gear range, and thereafter also the synchronization has to be achieved with a gear change of the main transmission. Shifting of the range group can hereby take a relatively long time. A range shift is therefore often considered by the driver as disturbing and can also have an effect with regard to the dynamics of the drive operation.
Known from the WO2008/107318 A1 is a group transmission with a main transmission, a split group and a range transmission in which the main transmission and the following range group are designed as unsynchronized and the upstream split group is synchronized. The synchronization device of the split group can be connected and disconnected from the drive motor by a separation clutch. The range group is equipped with a simple planetary gear set which can be shift between a low speed drive step, in accordance with the planetary gear ratio, and a high-speed drive step, in accordance with a direct coupling between the main transmission and output, as well as a neutral position.
A shift sequence diagram for this gearbox provides that, during a range shift in the main transmission and range group, in each case a change between two gear ratios steps takes place, wherein the range group and the main transmission can be shifted in opposite directions. During the shift and after disengagement of the separation clutch and the load reduction at the drive motor, the range group is shifted into neutral, so that the entire group transmission during the gear change is at least partially separated, and therefore completely free of load, from the drive motor and from the drive train at the output side, meaning from the wheels of the vehicle. Thereafter, the rotating free masses of the main transmission and the split group are slowed by means of a transmission brake to a reliable, shiftable low rotational speed for the disengagement. At the same time, the rotational speed of the motor starts to be adjusted to match the synchronous rotational speed of the target gear. The change of the gear ratio step in the main transmission and the change of the gear ratio step in the split group happen then in parallel. Thereafter, by at least partially engaging the separating clutch, the rotational speed of the free masses is accelerated to the rotational speed of the motor, in the direction parallel to the synchronous rotational speed of the target gear, and then the claw shifted range group is synchronized via a transmission main shaft. Thereafter, the new gear ratio step of the range group is actuated whereby, during a complete engagement of the separating clutch, the load increase of the drive motor starts and the gear change is therefore completed.
Known from EP 1 565 674 B1 is a group transmission having a main transmission, a split group and a range group, in which the main transmission is manually shifted by means of a hand shift device, and the gear ratio of the split group and the range group are preselected via the hand shift device. During a range shift, the recognition of the neutral position in the main transmission initiates the shifting of the range group. During the shift of the range group, the target gear ratio is selected in the main transmission, wherein servo assistance for shifting of the gear ratio step in the main transmission is delayed, and is preferably released only after shifting of the range group, to reduce strong shift sounds and the wear and tear.